One idea could be a character reincarnating as a powerful supernatural being in a different era and having to adapt to new powers and a new world order. For example, a modern - day human reincarnates as a vampire in the Victorian era and has to deal with the strict vampire society rules while also trying to figure out their new vampiric abilities.
Maybe a soul of a slain supernatural creature reincarnates into a human body. The human then starts having strange dreams and memories that lead them to discover their past life. They gradually regain their old powers and have to use them to either protect the world from an ancient evil that they once knew, or to redeem themselves for past misdeeds in their previous life.
A fairy Sim could lose their wings in a magical accident. They then have to go on a quest to find the magical ingredients to restore their wings. Along the way, they meet other supernaturals who either help or hinder them. This could involve a lot of exploration of the different supernatural worlds within the Sims 3 Supernatural game.
Another plot could involve a Pokemon with psychic powers so strong that it can create entire alternate realities. Trainers get trapped in these realities and have to figure out how to escape while also dealing with the strange Pokemon that exist there. This could involve a lot of exploration and mystery-solving.
One plot idea could be Ketch getting involved in a time - travel adventure within the Supernatural universe. He could go back in time and interact with the Winchester brothers at different points in their lives, changing the course of events in unexpected ways.
One plot idea could be that the Winchester brothers have to face a water - dwelling supernatural creature while on a boating trip. Maybe it's a mermaid - like being that lures sailors to their doom, and they have to figure out how to stop it without getting trapped in its watery lair.
One idea could be that Mjolnir has a connection to an ancient supernatural being in the Supernatural universe. For example, it might be the key to sealing away a powerful demon that was once imprisoned by the Norse gods. The Winchester brothers could find out about this connection and have to protect Mjolnir from various creatures who want to use it for their own evil purposes.
To create believable characters in supernatural reincarnation fanfiction, start with their flaws. Just like in real life, no one is perfect. If a character was a powerful sorcerer in their past life and is now reincarnated as a normal person, they might be frustrated by their lack of power and this could lead to arrogance or a sense of entitlement. Their dialogue should also reflect their past and present. They might use old - fashioned language from their past life's era on occasion, or refer to things that only a supernatural being would know. And don't forget about their growth. As the story progresses, they should evolve based on their experiences in both their past and current lives.
One idea could be an angel family dealing with a new threat from a powerful dark entity. The angels have to put aside their internal differences to protect the world. For example, an angel family that has a young and inexperienced angel who discovers a unique power that could be the key to victory but they need to train the young one fast.
One interesting plot idea could be that the Flashpoint event somehow creates a rift that pulls characters from Supernatural into the Flashpoint universe. For example, Sam and Dean Winchester could be chasing a demon that escapes into this new, altered reality. They'd have to deal with the different laws of physics and the presence of superheroes like Flash. Another idea is that a powerful supernatural being from the Supernatural world causes the Flashpoint event in the first place, perhaps as part of a larger plan to reshape all of existence. And a third idea could be that a character from Flashpoint, like Iris West, has some latent supernatural ability that is awakened by the presence of the Winchester brothers, and they have to figure out how to control it before it causes chaos in both their worlds.
One idea could be that the Winchesters end up in Star City and team up with the Arrow to fight a demon that is using technology to cause chaos. Maybe the demon has possessed some of the tech - savvy villains in the Arrowverse.
One idea could be a story where a character is given a second chance at life by the grace of God. For example, a hunter who was killed comes back to set things right in the Supernatural world. Another could be a situation where God's grace is used to purify a demon instead of just exorcising it, leading to a whole new kind of relationship between the hunters and the reformed demon. Maybe God's grace could also be the key to opening a portal to a dimension where all the lost souls of hunters are trapped, and the main characters have to rescue them.